This invention was conceived because of a pressing need to package memory die more compactly to support the development of a wafer scale vector processor (WSVP). Each element in the WSVP being developed required 16 memory die and two processor die. If a conventional two dimensional HWSI were used in the processor, the element would require an area of about 2".times.2", of which about 16/19 would be occupied by memory die. This would result in interconnects as long as 4" unacceptably slowing down the memory to the CPU interface. The objective is to place at least 4 of the processor elements into the 2".times.2" area, not just one.
The "sugar cube" memories of the type developed by Irvine Sensors Corporation and described in U.S. Pat. Nos. 4,525,921 issued to Carson et al, and 4,617,160 issued to Belanger et al, is a system of stacked memories mounted on top of the HWSI substrate. This approach saves area, but uses the height dimension, in a sub-optimal manner, wasting most of its capacity by introducing a "skyline" effect. Because of the "skyline" effect, this arrangement inhibits the stacking of HWSI modules directly on top of each other.
While the prior art can stack chips atop the substrate, this invention uses a patterned overlay HWSI, and places the chips into wells custom etched into the substrate and then interconnects the chips with planarized (flat) film placed over the top of all the chips. These are then placed into the HWSI substrate, providing the desired density while preserving surface planarity. This allows the substrates to be stacked, an advantage lost if memory cubes are mounted on top of the substrate. This invention makes the stacked die so that their total thickness approaches that of a conventional chip. Hence, these chip-like stacks preserve substrate planarity which allows heat to be removed more efficiently from a stack of substrates since there are no air gaps in between layers.
Chip thinning down to 2 mils has been accomplished in the prior art. With an 80 mil substrate, 16 layer stacks of 4 mil die thick can be produced with methods well within the skill of the art. To demonstrate the technology, however, the invention was reduced to practice and is shown herein as using four 10 mil thick die.